Load safari or google chrome if you have it and open youtube in one of those.
navigate to video, select 'share' select 'embed' then select 'show more'.
select a size of 640 x anything, then copy the line of code above the video pic. It will look similar to...
iframe width="640" height="360" src="
https://www.youtube.com/embed/J-hNe1xBFdM?rel=0" frameborder="0" allowfullscreen/iframe
(With some brackets around it.) Select Mr H's. forum, make a new post or reply and just paste the line of code in.
you will probably need to go out of the message and return to it in order to see the video.